A high-carbon steel wire used for steel cord, saw wire for semiconductor cutting, hose wire, etc. is required to have a good wire drawability from the viewpoint of productivity in addition to a high strength and high fatigue properties. In this context, various steel wire rods and steel wires of high qualities conforming to the above requirements have heretofore been developed.
For example, Patent Literature 1 proposes a technology of improving the wire drawability and fatigue properties of a hard steel wire for cold drawing by forming tempered lower bainite in a structure before wiredrawing. According to the technology, excellent wire drawability and fatigue properties after wiredrawing are materialized by drawing a lower bainite structure that is considered to be suitable for wiredrawing from the shape of carbide. The work hardenability of a bainite structure is lower than that of a pearlite structure however and a final wire strength is only about 3,500 MPa.
Further, Patent Literature 2 proposes a technology of improving wire drawability and fatigue resistance after wiredrawing by controlling a total oxygen quantity and the composition and number of inviscid inclusions. By the technology however, a fatigue limit stress to a tensile strength is only about 0.3 and fatigue properties are not necessarily exhibited sufficiently.
Patent Literature 3 discloses a technology of improving the fatigue properties of a high-strength wire by controlling the aspect ratio of inclusions in the steel wire. According to the technology however, a fatigue limit stress to a tensile strength is about 0.3 at the most and a sufficiently high fatigue strength is not yet obtained like Patent Literature 2.
Patent Literature 4 discloses a technology of improving the strain aging embrittlement resistance of a high-strength high-carbon steel wire by forming amorphous cementite as lamellar cementite in the pearlite structure of the wire and controlling the strength of the wire in a range stipulated by a wire diameter and a carbon quantity. By the technology, it is possible to produce a thin high-strength high-carbon steel wire having an improved longitudinal crackability but satisfactory high strength and high fatigue strength are not obtained yet.
Meanwhile, Patent Literature 5 proposes a technology of improving wire drawability and twistability by controlling the size of a pearlite nodule and the maximum length of secondary phase ferrite. By the technology, it is possible to obtain a high-strength high-carbon steel wire excellent in wire drawability but satisfactory high strength and high fatigue strength are not obtained yet.